Method to synthesize and produce thin films by spray pyrolysis

ABSTRACT

Forming a film by spraying onto a heated substrate an atomized solution containing the appropriate salt of a constituent element of the film and a reducing agent at a concentration greater than 1 M and greater than 10 times the stoichiometric amount of reducing agent.

The Government has rights in this invention pursuant to Contract No.XS-9-8014-3 awarded by the U.S. Department of Energy.

BACKGROUND OF THE INVENTION

The invention relates to spray pyrolysis, which involves forming a filmby spraying onto a heated substrate an atomized solution containing theappropriate salts of the constituent elements of the film compound. Thechemical reaction occurs upon spraying on the heated substrate, and thenonconstituent elements of the salts are removed by volatilization alongwith the solvent, typically water. For example, U.S. Pat. No. 3,148,084discloses, along with other examples, the formation of CdS by spraypyrolysis, according to the following equation:

    CdCl.sub.2 +H.sub.2 NC(═S)NH.sub.2 →CdS+volatile products

Before the conception of this invention Messrs. Steven A. Lis, Harvey B.Serreze, and Peter M. Sienkiewicz proposed the process claimed in acopending patent application (Ser. No. 231,127 filed Feb. 3, 1981)involving placing an agent directly in a spray solution to cause avariety of desirable oxidation/reduction reactions to occur when thespray solution contacted the heated substrate. Subsequently, but alsobefore the conception of this invention, Michael R. Squillante suggestedthat highly soluble-organic acids would be desirable reducing agents,and improvements in the purity of the film compound were noted (Ser. No.231,128 filed Feb. 3, 1981).

SUMMARY OF THE INVENTION

In the course of efforts to further increase purity, I discovered thatusing an unusually high concentration of reducing agent (i.e., greaterthan 1 M) and much greater than a stoichiometric amount of reducingagent (i.e., greater than 10 times and preferably greater than 900 timesthe stoichiometric amount), unexpectedly resulted in the formation of ahigh purity film.

In preferred embodiments a constituent element of the film is reducedafter contacting the heated substrate; the constituent element is Te andthe film is CdTe; the salts used in the spray solutions are (NH₄)₂ TeO₄and Cd(OH)₂ ; a copper dopant ("dopant" is used herein in the sensedefined at p. 372 of Kittel, Introduction to Solid State Physics, 4thed., John Wiley & Son) material is also placed in the spray solution,and it changes in oxidation state after contacting the heated substrate;and impurities are reduced to a volatile form after contacting theheated substrate.

PREFERRED EMBODIMENTS

The preferred embodiments of the invention will now be described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation of a photovoltaic device made according to theinvention.

FIG. 2 is a diagrammatic fragmentary vertical sectional view of a solarheat absorbing panel made according to the invention.

FIG. 3 is a diagrammatic representation of the spraying apparatus andheating means useful for making the devices of FIGS. 1 and 2 accordingto the invention.

STRUCTURE AND APPARATUS

Referring to the embodiment of FIG. 1, there is shown photovoltaicdevice 10 comprising glass substrate 12, layer of indium tin oxide 14(approximately 1000 Angstroms thick) thereon, layer 16 of sprayedcadmium zinc sulfide (1000 to 2000 Angstroms thick) thereon, layer 17 ofsprayed cadmium telluride (2500 to 5000 Angstroms thick) thereon, layer18 of Aquadag (a dried graphite-in-water suspension) thereon, wire 20attached to indium tin oxide layer 14 and wire 24 connected to theAquadag by silver paint patch 22.

In the embodiment of FIG. 2 there is shown solar heat absorbing panel 25comprising glass substrate 26 and layer 28 (1000-2000 Angstroms thick)of Al₂ O₃ -Ag cermet thereon. Dots 29 represent regions of metallicsilver dispersed throughout the Al₂ O₃.

Referring to FIG. 3, there is shown spraying and heating apparatus 30having air-tight enclosure 32. Spray nozzle 34 (1/4 J with a 1050 SSfluid cap available from Spraying Systems Company) is supplied bysolution bottle 36 and source of nitrogen spray gas 38. The gas flow isregulated by valve 40, and the pressure is monitored from gauge 42.Substrates 44 (e.g., substrate 12 and layer 14 or substrate 26) areheated by heater 46 through tin bath 48. Nozzle 34 is located 12" abovesubstrates 44. External means controls the temperature of heater 46, andthe heater is mounted within enclosure 32 on insulating block 50. Accessto the heater is provided by pass-through 52 (shown diagrammatically),and valve 54 regulates supply of nitrogen purge gas through the interiorof box 32. Heat shields 56 are used to shield the solution and nozzlecontrol apparatus 34, 36, 40 from high temperatures and the vapors fromthe heating apparatus below it.

Manufacture

In making the photovoltaic device of FIG. 1, films 16 and 17 are sprayedonto indium tin oxide coated glass (substrate 12 and layer 14 of theFIG. 1 device, available from Pittsburg Plate Glass Inc.) using the FIG.3 apparatus. First spray solution I for layer 16 and spray solution IIfor layer 17 are prepared by making aqueous solutions including theingredients set forth in the following table at the indicatedconcentrations.

                  TABLE 1                                                         ______________________________________                                                       Concentration in                                               Ingredient     Solution (M)                                                   ______________________________________                                        Solution I                                                                    CdCl.sub.2 . 2.5 H.sub.2 O                                                                   0.0167                                                         ZnCl.sub.2     0.0083                                                         Thiourea       0.025                                                          In(OH).sub.3   0.00025                                                        Formic Acid    9.62                                                           Solution II                                                                   Cd(OH).sub.2   0.021                                                          (NH.sub.4).sub.2 TeO.sub.4                                                                   0.085                                                          Cu.sup.++      0.00064                                                        Formic Acid    9.62                                                           ______________________________________                                    

The cadmium hydroxide is prepared by dissolving 20 gm of reagent gradeCd(NO₃)₂ in 100 ml distilled H₂ O and titrating this with a saturatedNaOH solution to complete precipitation of Cd(OH)₂. The precipitate isfiltered, washed several times with H₂ O and dried at 80° C. for 16hours.

The copper stock solution is prepared by etching copper shot (99% pure,available from Fisher Scientific Co), dissolving it in a slight excessof concentrated nitric acid, and diluting it to 0.05 M.

All chemicals are reagent grade and obtained from Fisher ScientificCompany, except for In(OH)₃, which is ultra pure and obtained from theAlpha Division of Ventron Corp., Danvers, Mass., and the (NH₄)₂ TeO₄,which is obtained in powdered form from Ventron.

In spite of the fact that a large amount of reducing agent is used, theTe in Solution II remains in the +6 oxidation state, allowing a higherconcentration of Te than would be possible if Te were in the highlyinsoluble -2 state.

The spray chamber within box 32 is purged using nitrogen gas. The volumeof the chamber is displaced three times with the nitrogen, and then thechamber is flushed continuously for five minutes more. The indium tinoxide coated glass substrates are cleaned by vapor degreasing withreagent grade trichloroethylene, and placed on the heated molten tinbath 48 and brought to a temperature between 325° and 550° C.,(preferably between 370° and 425° C., and most preferably of 400° C.).Thirty-five ml of solution I is sprayed onto the heated substrates withnozzle 34, the spray gas flowing at 2.0 ml/min at a pressure of 10 to 12psi, the liquid being pumped from bottle 30 by siphoning. The spray isdiscontinued by stopping the fluid flow from container 36. This resultsin the formation of layer 16 of ZnCdS on top of layer 14, the followingreaction occurring on the heated substrate: ##STR1## The indium isapparently reduced from the +3 state to metallic indium upon spraying onthe substrate, and this metallic dopant makes the layer more conductivewithout the necessity of a post-deposition annealing step.

Immediately following the spraying of solution I, 200 ml of solution IIis sprayed onto substrates 44 under the same spray and temperatureconditions, the following reaction occurring on the heated substrate:##EQU1## The tellurium is reduced to the -2 state, allowing it tocombine with the cadmium. The Cu⁺⁺ is apparently reduced to metalliccopper, thereby making the layer more conductive. Again, apost-deposition annealing step is not required.

The presence of the reducing agent in high concentration in bothsolutions I and II also induces removal of impurities by converting themto volatile reaction products, and nitrogen, an inert element, is usedas both the spray and ambient gas to avoid the extraneous introductionof impurities from the atmosphere.

The substrates are removed from the heater within three minutes afterspraying of solution II. The substrates are cooled in the purgedatmosphere for five minutes, and then removed from the chamber throughpass-through 52.

The Aquadag-E suspension (25% graphite in water; available from AchesonColloids Company) is applied to the upper surface of the sprayedsubstrates with a wood applicator and dried at room temperature. Silverpaint 25 is applied to the surface of the Aquadag-E layer 18, and copperwire lead 24 is attached to the paint 25 for external electricalconnection. The silver paint should be dried for several hours at roomtemperature. A second copper lead 20 is connected directly to the indiumtin oxide 12 using indium solder after scraping off some of layers 16and 17.

The cadmium concentration in the solution II is greater than thatnecessary to react with the tellurium because cadmium has been found tobe more volatile than the tellurium during spraying onto the heatedsubstrate.

Formic acid is a particularly useful reducing agent, as it is highlysoluble in aqueous solutions, and this allows increasing theconcentration of the reducing agent to the high concentrations that havebeen found necessary to obtain the desired, virtually completeoxidation/reduction reactions occurring on the heated substrate surface.Acetic acid is a highly preferred alternative. In general, it ispreferred that the reducing agent be a highly-water-soluble organicacid.

The formic acid concentration should be greater than 1 M and alsogreater than ten times the stoichiometric amount to cause the desiredoxidation/reduction reactions to occur upon contacting the heatedsubstrate.

The Al₂ O₃ -Ag cermet of FIG. 2 is made by the same procedure as theFIG. 1 device by spraying solution III (Table 2) onto glass substrate26, using the same spray, temperature, and nozzle conditions describedabove.

                  TABLE 2                                                         ______________________________________                                        Solution III                                                                                 Concentration in                                               Ingredients    Solution (M)                                                   ______________________________________                                        Al(NO.sub.3).sub.3 . 9H.sub.2 O                                                              0.003                                                          AgNO.sub.3     0.0015                                                         Formic acid    9.62                                                           ______________________________________                                    

Once again the chemicals are all reagent grade and are available fromFisher Scientific Company.

Examples of other solutions which can be sprayed with the FIG. 3apparatus are presented in the following table:

                  TABLE 3                                                         ______________________________________                                                                  Concentration in                                    Solution    Ingredients   Solution (M)                                        ______________________________________                                        IV          Bi(C.sub.2 H.sub.3 O.sub.2).sub.3                                                           0.01                                                            (NH.sub.4).sub.2 TeO.sub.4                                                                  0.0150                                                          Formic acid   9.62                                                V           Zn(N.sub.3).sub.2                                                                           0.21                                                            (NH.sub.4).sub.2 TeO.sub.4                                                                  0.21                                                            Formic acid   9.62                                                VI          AgNO.sub.3    0.1                                                             Formic acid   9.62                                                VIII        Cr(NO.sub.3).sub.3                                                                          0.1                                                             Formic acid   9.62                                                The reactions appear to be as follows:                                         ##STR2##            Bi.sub.2 Te.sub.3 + volatile reaction                                                       (4)ducts                                    ##STR3##            ZnTe + volatile reaction products                                                           (5)                                         ##STR4##            Ag° + volatile reaction products                                                     (6)                                         ##STR5##            Cr° + volatile reaction products                                                     (7)                                        ______________________________________                                    

A constituent element may itself be used as a source of dopant by usingan excess of the material in the spray. For example, the Cd portion ofCdTe can be increased as is described in the following equation:##STR6## In the above reaction, in order to maintain an excess of Cd inthe film, a much larger excess of Cd salt must be used in the spraysolution.

In all of the above examples, the presence of a reducing agent in a highconcentration also serves to convert impurities to a volatile form.

Other Embodiments

Other embodiments are within the following claims. For example theprinciples outlined above will find application in using an oxidizingagent when it is desired to have a film element in a higher oxidationstate in the film than in the starting material. Also, other dopantswill work. The solvent need not be water; methanol is a preferredalternative.

Finally, the principles outlined above will apply to making a largenumber of other films, e.g., Hg_(x) Cd_(1-x) Te, Zn_(x) Cd_(1-x) S, CdS,CdSe, ZnS, ZnSe, GaAs, Ga_(x) Al_(1-x) As, InP, BN, Ni-Co-Cr alloys, Zn₃P₂, and ZnSnP₂.

I claim:
 1. In the process of making a thin film comprising preparing a solution with solute elements including a salt of a first constituent element of said film and spraying said solution onto a heated substrate to form a film on said substrate, the solute and solvent elements not constituting said film forming volatile reaction products after contacting said heated substrate, said solution also including a reducing agent in sufficient amount to change the oxidation state of at least one said solute element after contacting said heated substrate, the improvement wherein the concentration of said reducing agent is greater than 1 M and greater than 10 times the stoichiometric amount necessary to react with said one solute element.
 2. The improvement of claim 1 wherein the concentration of said reducing agent is greater than 900 times the stoichiometric amount.
 3. The improvement of claim 1 wherein said one solute element is a constituent element of said film.
 4. The improvement of claim 3 wherein said one solute element is said first constituent element and is tellurium, said solute elements further comprising a soluble inorganic compound of a second constituent element, said second constituent element being cadmium.
 5. The improvement of claim 4 wherein said salt of said first constituent element is (NH₄)₂ TeO₄ and said soluble inorganic compound of said second constituent element is Cd(OH)₂.
 6. The improvement of claim 4 or 5 wherein another said solute element is a dopant, said dopant changing in oxidation state after contacting said heated substrate.
 7. The improvement of claim 1 wherein said one solute element is an impurity that converts to a volatile form with said change in oxidation state. 